Though I wouldn't defend the K501s bass, I think the Golden Ears presentation of the measurements is quite different from that of Headphone.com, which unfortunately doesn't have the K501. If you take the baseline of -20 as 0, then the K501 has is only a -7 or -8 DB below 0 response at 40Hz on Golden Ears, which is quite comparable to many good bookshelf speakers/monitors, even ones that cost thousands of dollars.
By that comparison, there's only a 4-5 DB lower amount of bass than the K702, which has about -2 to -4 DB below 0 at 40Hz on the Golden Ears site (http://en.goldenears.net/8281). So in all, you lose a significant amount of bass compared to the supposedly higher class and newer model, but I still doubt that difference is damaging to the enjoyment of most acoustic music. Most people who went from the K501 to the K701/702 directly, found the highs to be a bit less charming on the latter model, and the soundstage to be almost unnaturally expansive. I personally prefer them for most music to my Sennheiser HD650s, despite the significantly larger quantity of bass in the latter.
To understand the difference in presentation see Golden Ears vs. Headphone first compare two graphs of the K702 and then see the Golden Ears graph of the K501:
So when you look at the K501, take into account that the way the bass roll off looks more dramatic than it would if it were on Headphone.com.
The Headroom measurements on Headphone.com (build a graph), seem to show the K702 with bass that is in positive territory (above 0) right down until 40Hz where it starts going south (versus around 70-80 Hz on Golden Ears). If the Headroom guys are getting about 4-5 extra DB in the bass, then one could plausibly assume that if they had measured the K501 it would have shown bass at around 0 down to about 80Hz and then only about -5 to -7 DB at 40Hz, which seems to be closer to what I am hearing depending on the amp. While it's still not amazing, it does show that the K501 is probably not that far behind it's (younger) big brothers in terms of bass performance.
And as an final point, the Golden Ears frequency measurement display of the highs appears a lot more dramatic than Headphone, where the K702 looks to have a huge spike around 2K, but on Headphone.com the measurements look nearly flat, give or take a few DB until about 9K where the steep roll off begins. So without getting out a calculator, the treble on the K501 looks a bit smoother on Golden Ears than the K702. If that is the case, you could expect that if Headphone.com did review the K501, it would have a bit less bass from 100Hz down to 0hz, and show nearly linear treble up to around 10K.
Note: I'm a historian not a scientist or engineer, so please take this with a grain of salt.
Edited by jdpark - 1/19/16 at 1:08am